Constitutive and plastic components of activity rhythms under interspecific competition in hermit crabs

Species coexistence requires niche differentiation. Temporal partitioning of activity can be a key mechanism to reduce interspecific competition. This study assesses diel locomotor activity rhythms of two coexisting intertidal hermit crab species, Calcinus californiensis and Clibanarius albidigitus, which compete for the same limiting resource: gastropod shells. We assessed activity rhythms and total movement under three social conditions (alone, with conspecifics, and with heterospecifics) to evaluate the influence of intra- and interspecific competition on activity patterns in both species. The timing of peak (acrophase) and minimum (bathyphase) activity remained unchanged across conditions, suggesting a constitutive, evolutionarily conserved trait. However, both species exhibited plasticity in overall activity levels, midline estimating statistic of rhythm (mesor), and rhythm amplitude. The subordinate species, C. albidigitus, reduced its total activity and slightly advanced the beginning of its activity in the presence of the dominant C. californiensis, likely as a strategy to minimize the cost of encounters with its competitor. Conversely, C. californiensis increased its activity in response to both intra- and interspecific competitors, extending its active phase into the early morning hours, consistent with its agonistic behavior and greater competitive ability. Our findings support the idea that activity rhythms integrate both fixed (circadian) and plastic (context-dependent) components. While rhythmic phase (timing) appears to be constrained by ecological and evolutionary factors, modulation of activity intensity provides a flexible mechanism for adjusting to social contexts. Thus, rhythm plasticity seems to contribute to temporal niche partitioning, facilitating species coexistence even when the timing of activity remains stable.